An electric device may comprise a protective mechanism for protecting the device against high electric currents. Such currents may arise, for example, in the event of a short circuit, accident or other kind of failure. The protective mechanism may be arranged to interrupt an electric circuit in which an overcurrent has been detected. An overcurrent is an electric current that is greater than a maximum allowed current. A simple example of a protective mechanism is a fuse that is blown when the electric current through the fuse exceeds a maximum allowed current. Electronic protective mechanisms also exist.
Defining the maximum allowed current for a given application can be challenging because some electric devices may draw a large current when first turned on and a considerably lower stationary current after conductors in the device have heated up. This phenomenon is usually due to the fact that the electric resistance of a conductor tends to change, e.g. increases, as the temperature of the conductor increases.
For example, the electric device to be protected may be an incandescent lamp. The incandescent lamp may, for example, be a halogen lamp. Before the lamp is turned on, the temperature and, thus, the resistance of the lamp's filament may initially be very low. At turn-on, the temperature of the filament may start to rise from the ambient temperature. As the initial resistance may initially be low, a large initial current may occur when the lamp is turned on. A large initial current into a load upon turn-on is referred to as an inrush current. An inrush current may be many times (e.g. ten times) greater than a nominal current. The nominal current may be defined as the current through the load when the load has reached a stationary temperature. The expressions nominal current, stationary current, and steady state current may be interchangeable. Both the inrush current and the steady state current may depend on the voltage applied at the lamp. The voltage applied at the lamp may, in turn, be a function of a supply voltage. The supply voltage may notably be a direct current (DC) voltage, e.g. provided by a battery. A DC voltage is a voltage which remains substantially constant over a prolonged period, e.g. over a large number of cycles during which the load may be switched on and off. A protective mechanism should allow the inrush current to flow in the load, e.g., in the wiring harness, but only for a specified time, e.g., not longer than one hundred milliseconds after switching the lamp on.
International patent application publication WO 2006/111187 A1 (Turpin) describes a current driver circuit having a current limit that is continuously or intermittently adjusted. This adjustment may be based on the change of the load impedance over time, which may be substantially equivalent to a temperature change.